Present rotational internal combustion engines, such as diesel engines, are known to be relatively inefficient. At full load, the maximum efficiency of converting the heat of combustion into brake work is only about thirty-six percent, and this efficiency drops as the load and RPM change from optimum conditions. The losses which occur in rotational engines are attributable in part to the rotational manner of operation of such engines. Since the angle between the piston connecting rod and a radius to the center of the crankshaft is constantly changing, the proportion of the combustion pressure applied directly to the load also varies. In addition, such engines experience further losses during idling when the demand of the load is stopped at intermittent intervals. Multiple cylinders are provided for smoothness of operation, and this gives rise to a larger cylinder wall area through which heat is lost to the cooling water which must circulate around the exterior of the cylinders. The multiplicity of cylinders also increases friction losses. Furthermore, a large amount of heat energy is exhausted into the atmosphere.
Internal combustion engines have been used to operate hydraulic pumping systems. For example, in U.S. Pat. No. 1,083,568, a rotational internal combustion engine is used to drive a hydraulic pump. A starting mechanism is provided to start the rotational engine in response to a drop in the hydraulic pressure stored in the system. U.S. Pat. No. 2,334,688 discloses an internal combustion pump having a mechanically linked combustion piston and compression piston. U.S. Pat. No. 3,986,796 discloses an integral piston which functions as a compression piston, combustion piston and a bounce piston. U.S. Pat. No. 4,115,037 discloses a 2-cycle, internal combustion engine driving a hydraulic pump, there being a mechanical linkage between a combustion piston head and a compression piston head. U.S. Pat. No. 3,751,905 discloses a steam generating apparatus including dual pistons which are moved in the compression stroke toward a central combustion zone by fluid pressure from an accumulator, U.S. Pat. No. 2,352,267 discloses the injection of water into a combustion cylinder during combustion.
It has also been recognized that fixed displacement hydraulic motors are efficient means for transferring energy into rotational form. However, attempts to vary the torque applied to a load by such a hydraulic motor have generally taken the form of reducing the pressure of fluid supplied to the motor by using flow dividing valves, throttling valves and the like. Such devices result in the dissipation of energy and a reduction in efficiency.
Energy storage in hydraulic pumping and motor systems is known. In U.S. Pat. No. 3,922,854, a drive motor drives a primary fluid pump, and also drives an auxiliary pump which can be used to accumulate fluid pressure during times of low demand for later direct application to the load. U.S. Pat. No. 3,157,996 and U.S. Pat. No. 3,990,235 each disclose systems in which a drive engine drives a pump which drives hydraulic motors to turn a load, and in which an accumulator is provided in the fluid conduit between the pump and the motor.